A GARDA investigation has established that the details of six Irish passports were replicated for use by alleged members of a Russian spy ring.

The investigation, which is being carried out by detectives from the Special Branch, is trying to pinpoint where the details were copied and then inserted into the forged documentation.

The inquiries also involve officials from the Department of Foreign Affairs and the Passport Office and follow a tip-off to the authorities from the FBI.

The fraudulent use of the passports was discovered when the FBI smashed the spy ring, which was based in the US.

But it is not yet confirmed whether all of the forged documentation had been used by the spies.

One of the passports belonged to a volunteer with Irish charity To Russia With Love named as Kathryn Sherry and two others to a married couple in Co Donegal. All had all been granted visas at the Russian Embassy in Dublin.

After gardai have completed a file, it will be studied by senior officials from the Department of Foreign Affairs and, if the Russians are clearly implicated in the forgeries, a decision will be taken on whether diplomatic action should be taken.

Earlier this year, the Irish Government and public were outraged at the use of forged Irish passports by Israeli agents, who were alleged to have murdered Hamas activist Mahmoud al-Mabhouh in a Dubai hotel.

As a result of investigations by the Garda and the findings of police in other countries, the Government concluded that an official Israeli agency had been responsible for the "misuse and, most likely, manufacture of the eight fake Irish passports".

An Israeli diplomat was expelled from Dublin in retaliation for the passport abuses.

In the latest investigation it has been established that the six Irish passport holders had been granted visas at the Russian Embassy in Dublin.

But it is not yet clear how and when the details of the passports were copied and reproduced on the forgeries.

In both Israeli and Russian cases, the passports were of a type produced before the introduction of new design and security standards in 2005.

These included a more secure personal details page, made of a polycarbonate substance, containing the personal details and photograph of the holder.

The polycarbonate makes it virtually impossible to reproduce the page without sophisticated production processes or to alter it without being detected by standard examination techniques.

Garda Commissioner Fachtna Murphy said yesterday that the Garda were working close with US law enforcement agencies in investigating the fraudulent use of a small number of Irish passports.

"In the most recent case it is six, but any misuse of Irish passports is an issue for us and the Government and we take it very seriously."

But he warned that the nature of the investigation made a successful outcome more difficult. Some of the passports had been used internationally, and cross-border involvement created more obstacles for the investigation.

However, he said the changes that had been introduced to passports since 2005 had significantly improved their security.

Justice Minister Dermot Ahern said the new biometric passport was regarded as "the best in the world" and he understood that all of the passports now under investigation related to those issued under the old regime.